Light Architecture 2 By Urs Recher _TOP_
Cloridan Drakh
In the second, revised edition of his book, the author explains his personal ideas and experiences on the subject of light. Contrary to the first edition, some pictures have been replaced and the print quality has improved decisively.
This impressive and beautiful guide features in-depth tutorials based on Urs Recher's photographs, each illustrating a different lighting setup. The book contains 50 different examples, covering studio, mixed light and daylight shots. The visuals alone provide all the guidance one might need to recreate the shots, and the content is easily accessible for amateurs and professionals alike.
Emerging photographers and enthusiastic young photography students had an excellent learning experience through the informative lighting seminar by Urs Recher at SUNSTUDIOS in both Melbourne and Sydney.
Participants experienced different ranges of studio lighting set ups using the extensive Broncolor lighting range. Participants also obtained valuable insights into professional studio lighting techniques for high-end commercial and fashion photography while enhancing their general photography skill set.
Urs demonstrated several applications with examples of Broncolor lighting modifiers for still life, beauty sets and action photography. He guided the participants to the characteristics of several Broncolor light shapers, the quality of the light they produce, and how to adapt to different subjects and environments.
Urs Recher is professional head photographer and a factory trainer at Broncolor Switzerland. His large experience and photographic and lighting expertise is well-known by professional photographers and customers. Urs also has been renowned with his book "Light Architecture 1 & 2" with many examples of application and set up using the Broncolor light.
An appealing concept in synthetic chemistry is photo-induced catalysis; where dormant complexes become catalytically active upon activation with light. The ruthenium-based olefin metathesis complexes founded on the original Grubbs catalyst have probably been one of the most widely studied families of catalysts for the past 25 years. Greater stability and versatility of these olefin-metathesis catalysts has been achieved by careful design of the ligand sphere, including latent catalysts which are activated by external stimuli. This article describes our recent developments towards light-induced olefin metathesis reactions based on photoactive sulfur-chelated ruthenium benzylidene catalysts. Alternative chemical reactions, be it photo-induced olefin metathesis or other direct photochemical processes, by using light of different frequencies were studied in chemoselective chromatic orthogonal pathways. The lessons learned during the development of these reactions have given birth to selective photo-deprotection sequences and novel pathways for stereolithographic applications.
One of the leading families of thermally latent catalysts is the sulfur-chelated ruthenium benzylidenes [15], [24], [25]. These complexes share a cis-dichloro configuration around the metal center, with the chelated carbene in the apical position of a slightly distorted square-pyramidal structure. Furthermore, sulfur binds ruthenium stronger than oxygen, thus it was expected that these types of complexes would show enhanced latency when compared to the Hoveyda-Grubbs catalyst. Hence, in agreement with the earlier studies of Van der Schaaf et al. [26] and Ung et al. [27], which showed that the cis-dichloro arrangement caused a decrease in activity, the S-chelated benzylidene precatalysts exhibited complete latency towards RCM at room temperature and were active only at high temperatures [28], [29]. Similar latent behavior was observed in ROMP reactions: the particularly slow initiation of the S-chelated catalysts coupled with the very fast propagation, provided ROMP derived polymers with relatively low polydispersity, where the size of the polymer was controlled by the turnover number and not by the initiator to monomer ratio [30]. Recognizing the importance of the S-chelated influence in cis/trans isomerization and the activity of Ru precatalysts, a series of studies were performed to explore the boundaries of latency in this family of complexes [31], [32], [33], [34], [35], [36], [37], [38], [39].
Latent olefin metathesis catalysts activated by light are also being widely developed [40], [41], [42], [43], [44], especially for photo-induced catalysis [45], [46] and industrial applications [47], [48]. The seminal discovery that UV light can activate the sulfur chelated catalysts [49] created a new method for light controlled olefin metathesis reactions. In this article, we discuss in detail (i) the photo-activation process of sulfur-chelated ruthenium complexes; (ii) how the coupling of two orthogonal photochemical reactions (photoremovable protecting group PPG cleavage 254 nm/RCM 350 nm), enabled high regioselectivity towards five- or six-membered-ring frameworks by flipping the order of the reactions [50]; and (iii) a PPG insertion in a light activated S-chelated ruthenium catalyst that opened a doorway for stereolithographic applications and added a new tool for complex light-guided chemical processes [51].
Initially, it was envisioned that photodissociation of the sulfur-ruthenium bond [52] could be utilized to activate the latent catalysts. For this purpose several S-chelated ruthenium complexes were investigated, including some with aromatic substituents on the sulfur atom that absorb light at the UV-A region (Scheme 2). In all cases, the cis-dichloro isomer was obtained as thermodynamic product after 24 h reflux in DCM. Alternatively, lowering the reaction temperature, time and solvent polarity (from DCM to benzene) afforded the trans-dichloro isomers in high yields [31], [32], [33], [34], [35], [36], [37], [38], [39]. Notably, the length of S-Ru bond in the trans isomer is quite longer (2.48 Å) than its cis counterpart (2.34 Å), possibly due to the trans influence of the strong sigma donating NHC ligand [53], [54], [55].
Given that the cis isomer is more thermodynamically stable, heating can rapidly restore the equilibrium after irradiation; effectively converting almost all the active trans isomer to its inactive counterpart. Thus, irradiation of the mixture can generate the active complex, while short pulses of heating can deactivate it. An interesting switchable process was then planned by judicious use of light and heat. The activation cycle, UV irradiation of a solution of 4cis and DEDAM for 15 min at room temperature, was followed by 5 min heating to 80C. As expected, UV irradiation promoted RCM, while heating stopped the reaction because it quickly depleted the active isomer. Notably, a 5 min heating period at 80C without UV irradiation produced a negligible conversion, but was enough to remove all visible signs (NMR) of the trans isomer from solution. Further UV irradiation formed new 4trans species and resumed reaction conversion, while another round of short heating stopped it. This protocol allowed for precise quantities of DEDAM to be ring-closed at every activation/deactivation step as shown in Fig. 1. It is quite particular of this system that the activation is done by light and the deactivation by heat. Naturally, prolonged periods of heating constantly generate trace amounts of active catalyst which in the presence of substrate are consumed and continue to promote metathesis reactions, as observed in the thermally latent catalysts previously mentioned.
Thus, by following the chromatic orthogonality principles a novel regioselective synthetic pathway towards dihydrofuranes or dihydropyranes could be carried out using only the order of wavelength irradiation as the guiding force (Scheme 7). Pathway A started by irradiation of 10 at 350 nm to afford mainly five membered RCM products, followed by irradiation at 254 nm to produce 11a as the major product. Alternatively, pathway B began with the irradiation of 10 at 254 nm to afford alcohol 9 and then irradiation at 350 nm delivered the the six-membered ring RCM product 11b with good selectivity (6:1). This is a unique example where chromatic orthogonality is used in a light activated catalytic sequence together with a photolabile protecting group to achieve a remarkable regioselective synthesis.
Irradiation of a DCM solution containing diethyl diallylmalonate (DEDAM) and 13cis (5%-mol) at 254 nm (UV-C) and 350 nm (UV-A) expectedly afforded different results (monitored by 1H NMR). Irradiation with UV-A light resulted with 95% conversion of DEDAM to the cyclized product, whereas irradiation with UV-C light did not promote ring closing metathesis (RCM), and only unreacted DEDAM could be seen (Scheme 8a). Interestingly, further irradiation with UV-A (after the sample had been treated by UV-C) did not promote RCM, suggesting that all 13 had been depleted by UV-C irradiation. A control experiment was also carried-out using the following irradiation sequence: a solution containing DEDAM and 13cis (5%-mol) in DCM was irradiated by UV-A light until 50% conversion was reached. Then the solution was irradiated by UV-C, and immediately after with UV-A; however no further progress in RCM could be detected (Scheme 8b). In contrast, when catalyst 3cis was used, the irradiation with UV-C did not fully decompose the catalyst and further irradiation with UV-A did bring about RCM of DEDAM (Scheme 8c). These experimental results suggested that the photocleavage of the supersilyl protecting groups from precatalyst 13cis resulted in catalyst deterioration and decomposition.
The activation/deactivation protocol could also be used for other types of olefin metathesis reactions, especially in applications dealing with polymerizations [89]. Thus, the photoinduced ring opening metathesis polymerizations (PROMP) [45], [90], [91] of exo-dimethyl-5-norbornene-2,3-dicarboxylate norbornene (14) and cyclooctene (15) were studied by using the appropriate UV irradiation sequence. In both examples, irradiation with UV-A light of the monomers with 13cis in CH2Cl2 afforded complete polymerization (Scheme 9). The afforded polymers had high molecular weights and low polydispersities. However, if UV-C light (254 nm) was applied in the first step, the catalyst was permanently inhibited and no polymer was obtained.
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